Ductile and tough high strength steel



Patented Dec. 9, 1958 DUCTILE AND TOUGH HIGH STRENGTH STEEL Samuel J.Rosenberg and Carolyn R. Irish, Washington, D. C., assignors to theUnited States of America as represented by the Secretary of the Navy NDrawing. Application March 19, 1957 Serial No. 647,167

5 Claims. (Cl. 75-128) (Granted under Title 35, U. S. Code (1952), sec.266) The invention described herein may be manufactured and used by orfor the Government of the United States of America for governmentalpurposes without the payment of any royalties thereon or therefor.

This invention relates to high strength alloy steels possessingsufficient ductility and toughness to permit satisfactory use instructural applications.

In order to meet the great variety of uses for steel, many specialsteels suited to different uses have been developed. in structuralapplications, for example, it is important to have a steel possessing ahigh degree of strength.

However, in structural steels now in use, while many possess adequatestrength, where extreme strength is involved there may be a definitedeficiency in toughness and ductility. The combination of chemicalelementsin the present invention develops high strength with sufficientductility for structural applications by applying a heat treatment to analloy steel composition in which the percentage of silicon is more thandoubled over that usually employed and a small percentage of titaniumhas been added, as indicated in the following composition Table I givingelement percentage ranges:

In general, usual furnace procedures are followed in making the steel ofthe above indicated alloy. Any of the usual type furnaces may beemployed as, for example, the open hearth furnace. It is important thateffective deoxidation of the furnace steel be made, good results beingobtained by using aluminum addition, to the amount of two pounds per tonof steel. After the ingot is obtained, it is heat soaked, forged androlled to the desired form according to routine procedure, and thensubjected to the following heat treatment.

The alloy is normalized by heating to about 1700 F. and holding at thistemperature for at least one half hour per inch of thickness of theworkpiece, this treatment being followed by cooling in air. In somecases where machining is required, because of deep hardenability, smalland even moderately sized sections may prove difficult to machine afternormalizing, in which cases the steel can be annealed by heating toabout 1625" F., followed by slow (furnace) cooling to about 800 F. at arate not exceeding 200 F. per hour and cooling in air below 800 F.

After the normalization step, the steel is hardened by heating to apoint above the upper critical temperature (AC3), as 1650" F., holdingat this temperature for at least one-half hour, and quenching by fluids,such as oil. The steel is then double tempered-by heating in twosuccessive periods to the 400 F.500 F. temperature range and holding thesteel in this range for at least one and one-half hours, after which itis air cooled.

It should be pointed out that while normalizing is a desirable heattreatment it is not essential for hardness, which depends on the heatingto 1650 F. followed by quenching and the double tempering steps.However, the preferred procedure includes normalizing to increaseuniformity of the steel.

The element percentage range of the alloy steel has been indicatedhereinabove; as typical of particular steels tested, examples are givenin Table II:

Table 11 Percentages Elements A B C D E 0 42 0 42 0 39 0 40 0 40 O 760.76 0. 0.72 0 72 1. 59 1. 59 1. 70 1. 50 1. 50 1. 83 1. 83 1 80 1. 80 180 0.79 0 79 0.86 0 83 0 83 0.25 0.25 O 32 0 30 0.30 0 041 O 041 0 10 008 0 08 O. 00 0.0022 0. 00 0. 00 0.0011

The composite elemental percentages of many steel specimens approach thefollowing values:

Table III Elements Percentages Carbon 0. 40 Manganese. 0. 75 Silicon 1.60 Nickel 1. 80 Chromium 0, 85 Molybde 0. 30 Titanium 0. 10

These steels, produced by the indicated process including thenormalizing, hardening and double tempering steps, with criticaltemperatures at about 1600" F.-Ac 1350 F .Ar 540 F.Ms, and with doubletempering at successive time periods of one and one-half to four hours,have the properties as indicated in Table IV, which follows, averagevalues being given:

The results are indicative of a definitely high strength level andhardness, which of themselves might not be considered noteworthy; butwhen these properties are coupled with pronounced ductility, asevidenced by the reduction in area and elongation data, and with a highdegree of toughness, as indicated by the impact test data, the steelproperties are considered outstanding. The maintenance of toughness ontemperature drop to -40 F. from room temperature is also worthy of note,the lowest value for the impact test at either room temperature or 40 F.

Table V Property Value Hardness, Re 55.0 Ultlln te Tensile Strength, p,s. l 302 000 Yield Strength, p. s. L.

Elongation, percent in 1.4 inch 9.0

Reduction of Area, percent 38.2 Irnp1etl-t. Lb

Room Temt 6 Obviously many modifications and variations of the presentinvention are possible in the light of the above teachings. It istherefore to be understood that within the scope of the appended claimsthe invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A low-alloy, high-strength steel for structural applicationcharacterized by great hardness, good ductility and toughness when heattreated by quenching from above the upper critical temperature, anddouble tempered in the 400 F. to 500 F. range, said steel comprisingabout: 0.35 to 0.45% carbon, 0.50 to 1.50% manganese, 1.30 to 2.00%silicon, 1.50 to 2.50% nickel, 0.70 to 0.90% chromium, 0.20 to 0.40%molybdenum, 0.04 to 0.20% titanium, 0.000 to 0.0025% boron, and thebalance of said steel being essentially iron.

2. A low-alloy, high-strength steel for structural applicationspossessing pronounced great hardness, ductility and toughness containingabout: 0.35 to 0.45% carbon,

0.50 to 1.50% manganese, 1.30 to 200% silicon, 1.50 to 2.50% nickel,0.70 to 0.90% chromium, 0.20 to 0.40% molybdenum, 0.04 to 0.20%titanium, 0.000 to 0.0025% boron, and the balance of said steel beingessentially iron.

3. A low-alloy, high-strength steel for structural applicationspossessing pronounced great hardness, ductility and toughness containingabout: carbon 0.39%, manganese 0.75%, silicon 1.70%, nickel 1.80%,chromium 0.86%, molybdenum 0.32%, titanium 0.10%, and the balance ofsaid steel being essentially iron.

4. A low-alloy, high-strength steel for structural applicationspossessing pronounced great hardness, ductility and toughness containingabout: carbon 0.40%, manganese 0.72%, silicon 1.50%, nickel 1.80%,chromium 0.83%, molybdenum 0.30%, titanium 0.08%, boron 0.0011%, andbalance of said steel being essentially iron.

5. A process of heat treating a low-alloy steel as set forth in claim 2which comprises normalizing at a temperature above the upper criticaltemperature of said steel, hardening by quenching from a temperature inexcess of the upper critical temperature of said steel, and tempering bydual successive beatings to between 400 F. and 500 F. followed by aircooling, the sucfiessive heatings being held for at least one andone-half ours.

References Cited in the file of this patent UNITED STATES PATENTS1,519,388 Walter Dec. 16, 1924 2,283,299 Tisdale May 19, 1942 2,327,490Bagsar Aug. 24, 1943 2,339,368 Bagsar Jan. 18, 1944 2,673,147 BrezinMar. 23, 1954 2,791,500 Foley et a1 May 7, 1957 OTHER REFERENCESGrossrnann: Principles of Heat Treatment, 1940 Edition, pp. 143, 106-109and 110-125.

1. A LOW-ALLOY, HIGH-STRENGTH STEEL FOR STRUCTURAL APPLICATIONCHARACTERIZED BY GREAT HARDNESS, GOOD DUCTILITY AND TOUGHNESS WHEN HEATTREATED BY QUENCHING FROM ABOVE THE UPPER CRITICAL TEMPERATURE, ANDDOUBLE TEMPERED IN THE 400*F. TO 500*F. RANGE, SAID STEEL COMPRISINGABOUT: 0.35 TO 0.45% CARBON, 0.50 TO 1.50% MANGANESE, 1.30 TO 2.00%SILICON, 1.52 TO 2.50% NICKEL, 0.70 TO 0.90% CHROMIUM, 0.20 TO 0.40%MOLYBDENUM, 0.04 TO 0.20% TITANIUM, 0.000 TO 0.0025% BORON, AND THEBALANCE OF SAID STEEL BEING ASSENTIALLY IRON.